1. Field of the Invention
The present invention relates to a surge absorber used for protecting electronic components against abnormally high AC or DC voltage. More particularly, the present invention relates to a surge absorber using a plurality of gaps in series in a gap-type surge absorbing element.
2. Description of the Related Art
Gap-type surge absorbers are generally used for protecting electronic components connected to a circuit receiving a DC voltage, such as a cathode ray tube circuit (CRT). Abnormally high voltages, which are harmful to electronic components, may be created by static electricity or lightning surges. Gap-type surge absorbers, having a microgap-type discharge tube or a gap-type discharge tube, are conventionally used to protect the electronic components by controlling these abnormal voltages.
Referring to FIG. 7, a surge absorber of the prior art employs a microgap-type discharge tube 10 having a surge absorbing element sealed within an insulating tube 21. The surge absorbing element includes a columnar ceramic body 12 covered with a conductive film 11. Micro gaps 13 and 14 are formed about the circumference of columnar ceramic body 12 spaced apart in the longitudinal direction of columnar ceramic body. Cap electrodes 16 and 17, serving as terminal electrodes, are attached at the opposed ends of ceramic body 12. Lead wires 18 and 19 are connected to cap electrodes 16 and 17, respectively. An insulating tube 21 surrounding ceramic body 12 and cap electrodes 16 and 17 is filled with an inert gas. Lead wires 18 and 19 extend from cap electrodes 16 and 17 to permit connection to external circuits. As shown in the figures, the openings through which lead wires 18 and 19 pass are sealed using any suitable technique such as, for example, soldering.
Referring to FIG. 8, another surge absorber of the prior art employs a gap-type discharge tube 30 that has sealing terminal electrodes 31 and 32 at opposed ends thereof. Electrodes 31 and 32 seal an inert gas within an insulating tube 33. The region in insulating tube 33, filled with inert gas, between terminal electrodes 31 and 32, provides a discharge gap 34 to permit discharges to occur in the presence of excessive voltage applied to terminal electrodes 31 and 32.
Referring to FIG. 5, microgap-type discharge tube 10 or gap-type discharge tube 30 may be connected to a circuit as shown. A power source circuit 2 is composed of power source 6 of voltage V.sub.0, a resistor 7 of resistance R, and either a microgap-type discharge tube 10 or a gap-type discharge tube 30. Output terminals 3 and 4 of power source circuit 2 feed DC power to a using circuit such as, for example, a CRT 1.
Referring to FIG. 6, current-voltage characteristics of gap discharge tubes 10 and 30 are generally divided into a glow-discharge region and an arc-discharge region. In the glow-discharge region, a relatively low current flows through discharge tube 10/30. In the arc-discharge region, a relatively large current flows through gap discharge tube 10/30. The arc discharge is initiated by the application of an AC or DC voltage across terminal electrodes 16 and 17 that produces a current that exceeds the current in the glow-discharge region for microgap-type discharge tube 10, or across terminal electrodes 31 and 32 for gap-type discharge tube 30. Current-voltage characteristics between output terminals 3 and 4 of power source circuit 2 change as indicated by the solid line A in FIG. 6.
If the resistance value R of resistor 7 is reduced in an attempt to increase output current of power source circuit 2, holdover current (follow current) occurs at a point H on the low-resistance broken line B in FIG. 6. Holdover current is characterized by the continuation of discharge even after the applied voltage is reduced below the striking voltage. In order to prevent holdover current, it is conventional to reduce the output current of power source circuit 2 by increasing the value of resistance R as indicated by the one-point dash line C in FIG. 6, or by increasing the voltage level required to maintain the glow discharge as indicated by the two-point dash line D in FIG. 6.
In conventional circuits, holdover current results in ionization of the inert gas which remains in the device, and effectively provides a conduction path past the gap or gaps. The ionized gas provides a relatively low-resistance conduction path for the current such that the current can be maintained by a lower voltage than the original striking voltage. In an AC power supply, the ionized gas is capable of permitting resumption of conduction even after conduction is extinguished by the voltage passing through zero. In a gap-type surge absorber of the conventional construction, however, a change in glow discharge voltage causes a change in discharge starting, or striking, voltage, thus leading to inconveniences such as deterioration of the surge absorbing property and a corresponding deterioration in the protection provided to the electronic circuit. Consequently, it has been conventional to prevent holdover current by increasing the value of resistance R, thereby reducing the output current of power source circuit 2.